How is psychosocial/socioemotional development theory relevant in early adulthood (i.e., Erikson, et al)

Psychosocial and Socioemotional theories are relevant in every stage of development because they reflect years of empirical and quantitative research done exclusively to extrapolate what are the common behaviors that are considered within the range of the stage.

Although no theory is exact and expectations vary from person to person (which is what makes them "theories" as opposed to "laws" of development), the behaviors and needs commonly displayed by individuals of a specific age group...

Psychosocial and Socioemotional theories are relevant in every stage of development because they reflect years of empirical and quantitative research done exclusively to extrapolate what are the common behaviors that are considered within the range of the stage.

Although no theory is exact and expectations vary from person to person (which is what makes them "theories" as opposed to "laws" of development), the behaviors and needs commonly displayed by individuals of a specific age group are used to classify commonality as "norm". Anything that deviates from that norm would be considered deviant behavior, or a deviant need. This information helps psychologists and experts in human development/trait theory to establish what to expect from each age group, and what needs to be done to intervene when deviation is suspected.

A theory like Erickson's for example, concedes that there are challenges to be met in every developmental stage. This is a conflict that is meant to solidify the cognitive and behavioral skills that are acquired through our interaction with the environment. When the challenge is met functionally and in order, the result will be positive, and experience will be gained. If the conflict is either not attempted, not met, or failed and left alone, the opposite of the outcome will result.

As far as early adulthood, ages 20-35, Erikson argues that the developmental conflict is psychosocial stage 6: Intimacy vs. Isolation.

The contention is that, during this time period in an individual's life, there is a natural tendency to make connections that are are worthwhile, long lasting, and well-anchored. This is acquired through well-planned and healthy opportunities for interaction with peers within the same developmental range. This is why it is psychosocial (psycho-related to the mental conflict; social-related to the developmental interactive stage).

Now, keep in mind that each stages builds upon the skills learned from the previous one. So if each developmental conflict is resolved effectively, the next one will likely be successful.

By the time early adulthood comes, the individual had already gone through the first 5 stages of development. If all the conflicts are accomplished effectively, the individual should already enjoy the following personality traits:

Trust

Autonomy

Initiative

Industry

Identity

If the conflict is not resolved effectively then the outcomes, per stage are:

Mistrust

Shame/Doubt

Guilt

Inferiority

Role confusion

When stage 6 comes, Intimacy vs. Isolation, the need is to make meaningful connections that work effectively. If you go into a potential relationship with mistrust, self doubt, an inferiority complex, and confusion, there is little chance for any intimacy to occur. Most people drag that "luggage" of past, unresolved issues into new situations and wonder why nothing ever works. Thanks to the delineations of psychosocial theory one can look and see what behavioral patterns contribute to success or to chaos.